This invention relates to a method of manufacturing a thin film circuit, especially, a thin film RC (resistor/capacitor) circuit.
In recent years, to meet requirements for various circuit performances, a thin film circuit having capacitor elements and resistor elements formed on the same substrate has been widely used and successive steps involved in a usual method of manufacturing such a thin film circuit are shown at steps (a) through (j) in FIG. 1.
As shown at (a) in FIG. 1, a tantalum film 2 for capacitor is first formed by sputtering, for example, to a thickness of about 5000 .ANG. on a clean insulating substrate 1 made of glass, ceramic, alumina/ceramic or the like. After forming a capacitor pattern as shown at (b) in FIG. 1, a portion of the capacitor pattern is subjected to anodization to convert that portion into a dielectric layer 3 as shown at (c) in FIG. 1.
After that, a tantalum nitride film 4 is deposited on the entire surface of the substrate to a thickness of about 800 .ANG. as shown at (d) in FIG. 1 and thereafter a metal film 5 made up of three layers of titatium, palladium and gold respectively having thicknesses of about 200 .ANG., 2000 .ANG. and 8000 .ANG. is vapor deposited on the film 4 as shown at (e) in FIG. 1. Thereafter, the metal film 5 is etched to form a first conductor pattern as shown at (f) in FIG. 1 and then the tantalum nitride film 4 is patterned to form a resistor as shown at (g) in FIG. 1. The entire resistor is subjected to stabilizing heat treatment. Then a metal film 6 comprising two layers of nichrome and gold respectively having thicknesses of 500 .ANG. and 3000 .ANG. is vapor deposited to cover the stabilized resistor and the first conductor pattern as shown at (h) in FIG. 1. Then the metal film 6 is patterned by etching to form a second metal conductor pattern, thus forming a capacitor element and a resistor element on the same substrate as shown at (i) in FIG. 1. Finally, the resistor is trimmed through laser trimming process. A plan view as seen from above at (i) is illustrated at (j) in FIG. 1.
Such a prior art method is seen in "Integrated Tantalum Film RC Circuits" by W. H. Orr et al, PROCEEDINGS, 1970, 20th ELECTRONIC COMPONENTS CONFERENCE, PP602-612, particularly, FIG. 6. In this method, after completion of the resistance stabilizing heat treatment, the second metal conductor pattern is formed to provide an opposing electrode of the capacitor and connection between internal conductor patterns. However, to provide the internal connection pattern, a portion of the second metal conductor pattern is superposed on a portion of the first metal conductor pattern, resulting in a problem of step coverage.
This is caused by the fact that the thickness of the second metal conductor pattern covering the stepped portion of the first metal conductor pattern becomes extremely small owing to the shadow effect at the time of vapor deposition. This causes disconnection of conductor in the circuit during fabrication or operation of the circuit. This decreases the yield of the thin film circuit as well as the reliability thereof.
To prevent such disconnection, a method has been proposed in which the thickness of the second metal conductor pattern is increased for suppressing the step coverage. However, when the thickness of the second metal film is increased, the probability of local deposition of a teardrop like metal particle lump on the opposing electrodes of the capacitor at the time of vapor deposition would increase, thus not only decreasing the yield of satisfactory capacitors but also making it difficult to miniaturize the thin film circuit.